1. Field of the Invention
This invention relates to a novel high-sensitivity, high-density optical information recording medium which is adapted for the recording and readout of information by means of laser light. More particularly, this invention relates to an optical information recording medium which is suitable for the recording and readout of information through utilization of the change in reflectivity or transmittance which is produced in areas of the reading medium which have been exposed to high-density energy laser beams, said areas being local deformations or areas from which recording medium has been removed by fusion, decomposition or the like of the recording layer.
2. Discussion of the Background
Since laser-beam recording and reading does not involve the use of a contact-type scanning head, laser beam use has the advantage that the recording material is not worn or damaged by contacting elements, and much research work has been undertaken to develop a variety of optical information recording materials. Thus far, particularly in the fields of optical disks, laser printers, facsimile devices, and the like, many laser beam recording materials are known already. It is known that metallic materials such as Te, Bi, In, Ge, and the like, or their alloys or oxides can be used as laser-absorbing materials in the recording layer of a recording medium. However, metallic materials not only have high thermal conductivities and melting points, but they have large surface reflectivities so that laser energies cannot be utilized effectively. Another serious disadvantage of such metallic materials is their toxicity.
With regard to non-metallic optical information recording materials, it is known that organic dyes such as fluorescein, Sudan Black B, Congo Red, Sudan Blue, Rhodamine 6G, and the like can be used as laser-absorbing materials (see Japanese Patent Laid-Open Publication No. 56-55289 and Japanese Patent Publication No. 57-20919). Because the thermal conductivities of organic materials are as low as one-tenth to one-hundredth of those of metals, the heat generated by photo-thermal conversion can be effectively utilized and, moreover, since the dissipation of heat in horizontal directions in low, organic materials permit faithful recording of signals, i.e. high-density recordings can be obtained. However, because the known organic materials absorb mainly in the visible region of the spectrum, the compatible laser beam sources are limited to Ar.sup.+ laser (488 nm), He-Ne laser (633 nm) and the like. Thus, organic materials are not suitable for use with semiconductor lasers, which have oscillation wavelengths in the near infrared region (.about.800 nm) of the spectrum. Therfore, the organic laser absorbing materials do not permit the design of compact and light-weight equipment.
Examples of organic compounds which absorb in the near-infrared region of the spectrum include cyanine dyes, for instance, which have been disclosed in the literature such as Japanese Patent Laid-Open Publication No. 58-114989. However, cyanine dyes have the disadvantage that, since they are labile in the presence of water, oxygen or light, they do not permit the long-term stability of stored information.
As mentioned above, the organic optical recording materials which have been thus far proposed exhibit various drawbacks such as insufficient light absorptivity or reflectivity, inadequate durability, insufficient storage stability, and the like, and none of them fully meet the performance requirements for optical information recording materials. A need therefore continues to exist for recording materials of increased compatibility with laser light.